Chlorinated aromatic compounds are of great importance industrially. They are used as intermediates, in particular for the manufacture of agrochemicals, dyes and non-flammable dielectric liquids.
Numerous methods exist allowing the introduction of one or more chlorine atoms into an aromatic nucleus but most industrial processes use chlorination by chlorine, which chlorination is catalysed by metal chlorides. The catalysts which can be used for this reaction are AlCl.sub.3, TiCl.sub.4, SnCl.sub.4, SbCl.sub.3, SbCl.sub.5, ZrCl.sub.4, FeCl.sub.3, NbCl.sub.5, MoCl.sub.5, WCl.sub.6, GaCl.sub.3 or TeCl.sub.4. The most generally used are FeCl.sub.3, AlCl.sub.3, SbCl.sub.3 and SbCl.sub.5.
The chlorination reaction is generally carried out by introducing gaseous chlorine into a reactor containing the aromatic hydrocarbon to be chlorinated and the catalyst or catalytic system. The chlorination temperature is generally between 40.degree. C. and 120.degree. C.
The reaction is generally carried out at atmospheric pressure. On completion of the chlorination reaction, the reaction mixture, hereinafter denoted as reaction crude, is subjected to degassing under an inert gas which allows the HCl content to be brought from several thousands of ppm to several hundreds of ppm.
The degassed reaction crude is subsequently washed with water, optionally basified water, which makes it possible to remove the catalyst. However, this operation exhibits many disadvantages, because it is necessary in particular to dry the washed reaction crude. In addition, this washing operation generates effluents containing in particular metal chlorides and organochlorinated or chlorinated aromatic compounds, the latter being carried away during the said washing because of their solubility in water. Thus, for example, the solubility in water of monochlorobenzene is 150 ppm, that of dichlorobenzenes is 70 ppm and that of trichlorobenzenes is 20 ppm.
These organochlorinated compounds can also be carried away mechanically, in the form of vesicles.
Due to current regulations regarding the discharge of industrial effluents, it is essential to treat these aqueous discharges in order to lower their content of organochlorinated compounds as well as of metal chlorides.
These treatment operations are expensive and few effective solutions have been provided.
Thus, in U.S. Pat. No. 4,885,418, provision is made to pass a reaction mixture originating from the chlorination of aromatic hydrocarbons in the presence of metal chlorides over a wet anionic resin in the hydroxyl form.
Although this process exhibits the advantage of being able to remove most of the metal chlorides, it however exhibits many disadvantages. This is because it is necessary to very thoroughly degas the reaction mixture in order to remove hydrochloric acid before passing over the anionic resin. Subsequently, the regeneration of the resins is expensive and requires use of aqueous hydrochloric acid solution and then aqueous sodium hydroxide solution and inevitably results in aqueous effluents which have to be treated. In addition, this process is entirely silent on the removal of organochlorinated compounds.